Osmium Doping Improves Recording Media

Thin Films Have High Coercivity and Coercive Squareness

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In magnetic recording the continuing demand for ever increasing recording density has stimulated research on thin films of continuous magnetic materials suitable for the production of high capacity storage discs. Sputtered γ -Fe2O3 thin films are attractive for this application in view of their high coercivity and high remanent magnetisation, combined with their resistance to corrosion and wear, and a γ -Fe2O3 film containing small amounts of cobalt, copper and titanium has been developed. The function of these additions is to increase the coercivity of the film, to improve coercive squareness, to widen and lower the temperature range of the γ -Fe2O3 to Fe3O4 reduction—so making it possible to obtain uniform magnetic properties—and also to suppress grain growth during heat-treatment.

Now workers at the Ibaraki Electrical Communication Laboratory in Japan report that remarkable improvements have been made to the magnetic properties and microstructure of sputtered γ -Fe2O3 thin films when osmium is used as an additive element (O. Ishii and I. Hatakeyama, J. Appl. Phys., 1984, 55, (6), 2269–2271).

Films 0.1 to 0.2μ m thick have been prepared by reactive magnetron sputtering, the target being an iron plate to which osmium pellets were attached. Coercivity and coercive squareness increased with osmium content, to maximum values of 2100 Oe and 0.81, respectively. Osmium doping also brought about field-induced anisotropy which greatly increased the coercive squareness parallel to the easy axis, a figure of 0.96 being obtained with 0.88 to 5.2 atomic per cent osmium. Osmium also suppressed grain growth during preparation, giving crystallites about 400 Å in diameter which improves the signal to noise ratio, an advantage for increasing recording density and read back amplitude.